106 NUTRITIONAL PHYSIOLOGY 



ing the next transit of the blood through the lungs. Other 

 waste substances are gathered by the blood as it flows near 

 the active cells. The major part of these is destined for 

 excretion by the kidneys, and these organs receive so large 

 a share of blood that the entire volume must come under 

 their influence within a short space of time. A minor 

 fraction of the waste finds an outlet in the bile, the sweat, 

 and through the intestinal wall. 



It has already been pointed out that every organ has a 

 chemical constitution and a metabolism peculiar to itself. 

 Therefore each organ must make certain demands upon the 

 blood not exactly duplicated elsewhere. What is more 

 important, each organ gives rise to products unlike those 

 formed by any other part. In a number of cases these 

 products can be shown to have far-reaching effects. Their 

 existence has been mentioned in Chapter IV. Recognizing 

 the large part which they play in the economy of the organ- 

 ism, we may state at this point that an essential service 

 of the blood is the transportation of internal secretions. 



Another function of the blood, and one often overlooked, 

 is that of equalizing the temperature of different regions. 

 One is reminded of the arrangement of a heating system in 

 a house where a fire burns in a furnace and a circulation 

 of air, hot water, or steam disperses the heat through the 

 rooms above. Interruption of this circulation will allow 

 the upper stories of the house to cool off, while the base- 

 ment is overheated. In the living body heat production 

 is a function of all active tissues. The ancients believed 

 that it was the particular duty of the heart. This organ is, 

 in fact, distinguished for its rapid evolution of energy, but 

 it must be remembered that it is of small bulk. A much 

 larger share of the aggregate heat production is borne by 

 the skeletal muscles. These are supplemented to some 

 extent by the liver and the other great glands of the body. 

 Blood passing through a tissue which is undergoing lively 

 metabolism will have a higher temperature as it leaves than 

 it had when it entered. As it flows on it will communicate 

 some of this surplus heat to resting structures in which 



